Naphtha steam-cracking quench process

ABSTRACT

In quenching the hot gaseous effluent from a naphtha steamcracking operation employing transfer line heat exchangers, improved heat recovery is obtained by recycling a high boiling fraction, i.e., filtered steam-cracked tar bottoms, to the quench point in addition to quenching with a steam-cracked gas oil fraction recovered from the quench tower. The use of the highboiling fraction, i.e., tar bottoms, in conjunction with the steam-cracked gas oil fraction to quench the effluent which has previously passed through a transfer line heat exchanger, maintains the temperature of the effluent passing to the quench tower in the range of from about 500* to about 650*F. while insuring a liquid phase on the walls of the transfer line to prevent fouling, and thus allows additional high-level heat recovery in the quench tower.

[ 1 Dec.2, 1975 1 NAPHTHA STEAM-CRACKING QUENCH PROCESS [75] Inventor:Walter C. Kohfeldt, Madison, NJ.

[73] Assignee: Exxon Research & Engineering Co.,

Linden, NJ.

[22] Filed: May 30, 1973 [21] Appl. No.: 365,052

Related US. Application Data [63] Continuation of Ser. No. 119,485,March 1, 1971,

3,597,494 8/1971 Bigache ct al. 260/683 3,647,907 3/1972 Sato et a1260/683 3.676.519 7/1972 Dom et a] 260/683 Primary ExaminerDelbert E.Gantz Assistant Examiner-C. E. Spresser Attorney, Agent, or Firm-DonaldC. Caulfield [57] ABSTRACT In quenching the hot gaseous effluent from anaphtha steam-cracking operation employing transfer line heatexchangers, improved heat recovery is obtained by recycling a highboiling fraction, i.e., filtered steamcracked tar bottoms, to the quenchpoint in addition to quenching with a steam-cracked gas oil fractionrecovered from the quench tower. The use of the high boiling fraction,i.e., tar bottoms, in conjunction with the steam-cracked gas oilfraction to quench the effluent which has previously passed through atransfer line heat exchanger, maintains the temperature of the effluentpassing to the quench tower in the range of from about 500 to about650F. while insuring a liquid phase on the walls of the transfer line toprevent fouling, and thus allows additional high-level heat recovery inthe quench tower.

3 Claims, 1 Drawing Figure H MEDOE US. Patent Dec. 2, 1975 w &

W. 6. Ko'hfe/df INVENTOR BY ATTORNEY NAPIrITHA STEAM-CRACKING QUENCHPROCESS This is a continuation of application Ser. No. 1 19,485 filedMar. 1, 197], now abandoned.

FIELD OF THE INVENTION This invention relates to a method of quenching ahot gaseous effluent of steam-cracked naphtha products being transferredfrom a cracking coil outlet into a quench tower for separation of theproducts into fractions. More particularly, this invention relates to animproved steam-cracking quench process wherein the ef fluent from thefurnace is first passed through a heat exchanging zone and thereafterquenched with a circulating quench oil fraction, i.e., steam-cracked gasoil recovered as a distillate fraction from the quenching andfractionationzone. i.e., quench tower and with a high boiling fractionsuch as filtered steam cracker tar bottoms recycled from the quenchtower to the quench point Employing such a novel quench system allowsthe temperature of the effluent passing to the quench tower to bemaintained at a temperature in the range of from about 500 to about650F. while at the same time maintaining a liquid phase on the wall ofthe transfer line or order to prevent fouling thereof. Accordingly,high-level heat recovery is obtained by first passing the effluent fromthe furnace through a transfer line heat exchanger and, after employingthe quench system heretofore described, passing the effluent at apreferred temperature in the range of from about 525 to about 600F. intothe quench tower such that a significant amount of high-level heat isfurther recovered when the circulating quench oil fraction is removedfrom the quench tower, passed through one or more heat exchangers andthereafter employed as a quenching medium at the transfer line quenchpoint and in the quench tower itself.

DESCRIPTION OF THE PRIOR ART In steam-cracking virgin naphtha fractionsto produce high yields of C to C olefins and diolefins using crackingconditions at high temperatures and low pressures, quick quenching andfast separation of products are necessary. It is necessary to quench theproduct from the cracking zone; that is, chill, cool them suddenly andrapidly to a lower temperature to prevent or minimize side reactionswhich reduce yields of desired products and increase yields of undesiredproducts.

In the prior art there are numerous disclosures of different quenchingagents or mediums and among these are high-boiling hydrocarbons,low-boiling hydrocarbons, water, steam, and the like. In many casesfouling occurs at or beyond the quench point or region and the processmust be terminated to clean the equipment. Simultaneously, whennaptha-cracking operation eco nomics favor the use of transfer line heatexchangers to quench the effluent from the steam-cracking furnace, it isimportant to maximize the level of heat recovered in the quenchingprocess in order to generate high pressure steam for turbine drivers andthe like.

In conventional naphtha steam-cracking quench systems, the effluent isquenched with a steam-cracked oil distillate fraction recovered from thequench tower and recycled to the quench point. The use of this fractionas the quench oil lowers the temperature to a range of from about 5 toabout'600F., permitting substantial recovery of high-level heat, i.e.,recovering X BTU per hour as 125 to about 145 psig steam generation inthe quench tower equipment. However, when one or more transfer line heatexchangers are employed to initially cool the effluent from the furnaceand to recover a substantial amount of heat, i.e., about 0.85 X BTU/hr.as 1200 to about 1800 psig high-pressure steam, a considerably lowertemperature after oil quench must be employed in order to maintain theliquid phase on the walls of the transfer line passing to the quenchtower in order to prevent fouling and plugging of the transfer line.This operation lowers the temperature of the effluent in the transferline to from about 350 to about 425F. such that only a low-level heatrecovery is economical in the quench tower, i.e., 0.15 X BTU/hr. as 25psig steam.

SUMMARY OF THE INVENTION cordance with the instant invention, ahigh-boiling fraction such as filtered steam-cracked tar bottomsrecovered from the quench tower is recycled along with a steam-crackedoil distillate fraction recovered from the quench tower to the quenchpoint in order to maintain the temperature of the effluent in thetransfer line in the range of from about 525F. to about 600F. while atthe same time maintaining a liquid phase on the wall of the transferline passing to the quench tower in order to recover 0.85 X BTU/hr. asabout 1800 psig steam and 0.15 X BTU(hr. as to about psig steam. Thus,the instant quenching scheme allows the effluent to pass into the quenchtower at a sufficient temperature to provide for substantial high-levelheat recovery in the quench tower equipment and at the same timeprovides a liquid phase on the wall of the transfer line passing fromthe quench point to the quench tower in order to prevent plugging andfouling therein.

The manner of quenching the high temperature steam-cracked naphthaproducts and obtaining the quick separation of these products intosuitable fractions while allowing for an improved heat recovery will befurther understood by reference to the accompanying drawing.

Referring to FIG. 1, which shows a diagrammatic flow plan of the naphthasteam-cracking quench system of the instant invention, a hydrocarbonfeedstock 1 is passed by line 2 into a preheat and cracking coil locatedwithin the cracking furnace 3, wherein the cracking coil is exposed tohigh intensity radiant heat. The preferred hydrocarbon feedstock is anaphtha fraction containing principally C,-,C saturated aliphatichydrocarbons, i.e., paraffins or naphthenes, boiling principally in therange from about 100 to 350F. The feedstock may have a somewhat narrowerboiling range, e.g., in the range of from about 100 to about F.

A suitable proportion of steam passing by way of line 4 is added to thehydrocarbon feed to make the resulting cracking mixture contain fromabout 40 to about 65 mole steam, thus substantially lowering the partialpressure of the hydrocarbons. In the cracking coil located withinfurnace 3, the naphtha hydrocarbons mixed with steam are heated to anoutlet temperature in the range of from about l450 to about 1650F.,preferably in the range of from about 1500 to loO0F.

The total pressure of the cracked reaction mixture is in the range offrom about 1.5 to 3 atmospheres, and preferably less than 30 pounds persquare inch absolute. The residence time of the cracked reaction mixtureof steam and hydrocarbons in the cracking coil is in the range of fromabout 0.] to about 0.6 seconds, and more preferably for about 0.3 toabout 0.5 seconds. On leav ing the outlet of the coil, the crackedreaction of products is transferred by way of line 5 to transfer lineheat exchanger 6. The temperature (cot) of the reaction products leavingthe outlet of the coil is in the range of from about l450 to 1650F., andmore preferably from about l500 to 1600F. After passing through heatexchanger 6, the effluent is cooled to a temperature below about 850F.,and preferably to a temperature in the range of from about 650 to about8l5F. The heat recovered by the effluent passing through the heatexchanger generates steam in the range of from about 600 to about 1800psig steam, preferably from about 1400 to 1800 psig.

The effluent having been quenched in the transfer line heat exchanger toa temperature in the range from about 650 to about 815F. is passed byway ofline 7 to the quench tower 8. A steam-cracked oil fraction iswithdrawn from the lower portion of the quench tower 8 at a temperaturein the range of from about 450 to 550F. through line 9 and is passed bypump 10 through line 1] for cooling in heat exchanger 12. Heat recoveredin heat exchanger 12 generates steam in the range of from about 100 toabout 150 psig. The cooled oil distillate fraction leaving the heatexchanger 12 at a tern perature in the range of from about 350 to about400 F. through line 13 becomes divided into two streams. One stream ofthe cooled oil distillate fraction is passed through line 14 for furthercooling in the second heat exchanging zone 15. Heat recovered in heatexchanger 15 generates steam in the range of from about 10 to about 30psig. The oil distillate cooled in the heat exchanger 15 is passed byline 16 into the top section of the quench tower for partial cooling ofthe vapors flowing upwardly through quench tower 8. A sufficient amountof the oil distillate having a temperature in the range of from about350 to about 400F. is passed from line 13 into line 14 and is injectedat one or more points into line 7 to effect the lowering of thetemperature in the effluent passing from the transfer line heatexchanger to a temperature in the range of from about 525 to about 600F.The steam-cracked oil distillate fraction which is injected into line 7by way of line 14 has a preferred boiling range of from about 350 toabout 750F., and more preferably from about 450 to about 650F. The flowrate of the oil distillate from line 14 into line 7 is in the range offrom about 0.3 to about 0.8 parts by weight of the oil fraction per onepart by weight of the effluent from the furnace, the oil fraction havinga temperature in the range of from about 350 to about 400F.

The bottoms fraction is withdrawn from quench tower 8 at a temperaturein the range of from about 525 to about 600F. through line 17 and ispassed by pump 18 through line 19 to a filter 20. The bottoms productrecovered from quench tower 8 to line 7 comprises steam-cracked tarbottoms. The coke and other carbonaceous particles present in saidproduct are removed in the filter 20 and tower bottoms pass by way ofline 21 for cooling in heat exchanger 22. A sufficient amount of thecooled high-boiling fraction is recovered from heat exchanger 22 andpassed by way of line 23 through line 24- and injected into line 7 atone or more points in order to maintain a liquid phase on the wall ofline 7. The boiling point'of the material. i.e.. steamcracked tarbottoms passing by way of line 24 is in the range of from about 550 toabout 800F. The flow rate from line 24 into line 7 is in the range offrom about 0.01 to about 0.03 parts by weight of the bottoms productpassing by way of line 24 per one part by weight of the sum of theeffluent and oil distillate, the temperature of the high-boilingmaterial passing by way of line 24 being in the range of about 130 toabout 250F.

The mixture of the cracked products and the steamcracked oil fractionand high-boiling bottoms fraction passing by way of line 7 is preferablyintroduced into the bottom of the quench tower at a temperature in therange from about 525 to 600F.

gaseous vapors containing steam-cracked hydrocarbon products boilingbelow about 450F., steam, and hydrogen is taken overhead from the tower8 through line 25. This gaseous stream is passed by way of line 25 intocondenser 26 which is operated at a sufficiently low temperature tocondense out water and hydrocarbons having more than about 7 carbonatoms per molecule. The condensate is then passed by way of line 27 intoa separation tank 28 wherein the liquid condensate is settled so that alower water layer can be withdrawn to line 29 and condensed oil can bewithdrawn from an upper liquid layer through line 30. Uncondensedgaseous hydrocarbon products containing principally oletins anddiolefins having up to about 6 carbon atoms per molecule are withdrawnfrom vessel 28 to line 33 to be subjected to light ends processing i.e.,recover ethylene, propylene, butenes, butadienes and the like. A portionof the condensed oil withdrawn from settling drum 28 by way of line 30is passed by way of line 31 into the upper portion of quench tower 8 toform a reflux medium for the top of the tower.

The interior of quench tower 8 is equipped with plates 32 for obtainingcontact between the liquid and vapor but allowing for the fast flow ofmaterials. The necessary cooling is obtained in the upper part of thequench tower 8 by the injection of the cooled oil distillate fractionintroduced to the tower by way of line 16 and by introducing thecondensed oil introduced by way of line 31. Each of these streams isintroduced in suitable amounts and at the proper temperatures at severalspace points in order to obtain the optimum cooling and fractionation.The temperature of the vapors at EXAMPLE While various virgin naphthafractions may be employed as the feedstocks of the instant invention, apreferred feed contains hydrocarbons boiling principally in the range ofto 300F. The feed is cracked at temperatures of about 1500F. in thepresence of a sufficient amount of steam to make the hydrocarbon partialpressure about 14 lbs. per square inch absolute. The cracked productsleave the outlet coil at about 1500F. and are then introduced into thetransfer line heat exchanger. The cracked products are recovered fromthe transfer line heat exchanger at a temperature in the range of fromabout 650F. to 8l5F. The effluent is then quenched by a steam-crackedgas oil distillate fraction being removed from the lower section of thequench tower, and after passing through a heat exchanger is injectedinto the quench point or points at a temperature of about 375F., theproportion of steamcracked oil quench being about 0.5 times the weightof the admixed hydrocarbon products which are being quenched.Simultaneously, a steam-cracked tar bottoms fraction having a boilingpoint of about 750F. is removed from the bottom of the quench tower, andafter being filtered and passed through a heat exchanger such that thetemperature of the tar fraction is about 200F., is injected at saidtemperature into the quench points in the transfer line, the proportionof tar bottoms quench oil being 0.02 times the weight of the admixedcracked hydrocarbon products and steamcracked oil quench which arepassing in the transfer line to the quench tower.

The amount of steam-cracked gas oil quench and tar bottoms quenchinjected into the transfer line is sufficient to maintain thetemperature in the transfer line at about 525 to 600F., whilemaintaining a liquid phase on the wall of the transfer line in order toprevent coking and plugging of the transfer line. The quenched effluentis then introduced into a combined quenching and fractionation zone, thegaseous products being 7 cooled by being brought successfully intocontact with the cooled distillate fractions either removed or recoveredfrom the quenched tower. The gaseous stream that is recovered from thetop of the quenched tower is taken overhead and passed into a settlingdrum to separate out water, condensed oil, and uncondensed gaseoushydrocarbon products containing principally olefins and diolefins.

A sidestream may be withdrawn from the quench tower to remove cycle oilboiling in the intermediate range between the overhead and bottomsproducts. This may be necessary to control the boiling range of thedistillate oil to permit generation of 100 to 150 psig steam. Thesidestream is usually steam stripped to recover absorbed lighthydrocarbon products.

The above-described novel quench system results in recovering about 0.85X 1800 psig steam by passing the gaseous effluent from the furnace tothe initial transfer line heat exchanger. Additional heat is recoveredi.e., 0.15 X BTU/hr, in the range of from about 100 to about 150 psigsteam in initially cooling the steamcracked gas oil distillate fractionremoved from the middles of the quench tower, before recycling as aquench oil to the transfer line. Additionally, a lower upper part of thequench tower for cooling and reflux purposes.

What is claimed is:

1. In a process of treating a hydrocarbon feed stock in a steam-crackingfurnace at elevated temperatures and low hydrocarbon partial pressuresto form unsaturated hydrocarbon products wherein a quenching medium isintroduced into the effluent from the furnace and thereater passing saideffluent through a transfer line heat exchanger to the quench tower, theimprovement which comprises passing said effluent from the furnacethrough a transfer line heat exchanger zone to lower the temperature ofsaid products below about 850F. and thereafter contacting said productspassing to the quenching and fractionation zone with:

a. from about 0.3 to about 0.8 parts by weight of a steam-cracked oilfraction recovered from the quench tower per one part by weight of theeffluent from the furnace, said steam-cracked oil fraction having beenwithdrawn from the quench tower at a temperature in the range of fromabout 450 to about 550F. and therafter passed through a heat exchangingzone in order to recover high level heat in an amount in order togenerate steam in the range of from about to about psig; and

b. from about 0.01 to about 0.03 parts by weight of a bottom fractionrecovered from said quenching tower per one part by weight of the sum ofthe effluent and the steam-cracked oil fraction, said fraction boilingin the range of from about 500 to about 800F. to lower the temperatureof the products passing through the transfer line to the quench tower toa range of from about 525 to about 600F. in order to maintain a liquidphase on the wall of said transfer line.

2. The process as described in claim 1 wherein the hydrocarbon feedstockis a naphtha fraction boiling in the range from about 100 to about 350F.

3. The process of claim 1 wherein a portion of the steam-cracked gas oilfraction which has been cooled to a temperature in the range of fromabout 350 to about 400F. is passed through a second heat exchanging zonein order to further generate steam in the range of from about 10 toabout 30 psig. l

1. A PROCESS OF TREATING A HYDROCARBON FEED STOCK IN A STEAM-CRACKINGFURNACE AT ELEVATED TEMPERATURES AND LOW HYDROCARBON PARTIAL PRESSURESTO FORM UNSATURATED HYDROCARBON PRODUCTS WHEREIN A QUENCHING MEDIUM ISINTRODUCED INTO THE EFFLUENT FROM THE FURNACE AND THEREAFTER PASSINGSAID EFFLUENT THROUGH A TRANSFER LINE HEAT EXCHANGER TO THE QUENCHTOWER, THE IMPROVEMENT WHICH COMPRISES PASSING SAID EFFLUENT FROM THEFURNACE THROUGH A TRANSFER LINE HEAT EXCHANGER ZONE TO LOWER THETEMPERATURE OF SAID PRODUCTS BELOW ABOUT 850*F. AND THEREAFTERCONTACTING SAID PRODUCTS PASSING TO THE QUENCHING AND FRACTIONATION ZONEWITH: A. FROM ABOUT 0.3 TO ABOUT 0.8 PARTS BY WEIGHT OF A STEAMCRACKEDOIL FRACTION RECOVERED FROM THE QUENCH TOWER PERONE PART BY WEIGHT OFTHE EFFLUENT FROM THE FURNACE, SAID STEAM-CRACKED OIL FRACTION HAVINGBEEN WITHDRAWN FROM THE QUENCH TOWER AT A TEMPERATURE IN THE RANGE OFFROM ABOUT 450* TO ABOUT 550*F. AND THEREAFTER PASSED THROUGH A HEATEXCHANGING ZONE IN ORDER TO RECOVER HIGH LEVEL HEAT
 2. The process asdescribed in claim 1 wherein the hydrocarbon feedstock is a naphthafraction boiling in the range from about 100* to about 350*F.
 3. Theprocess of claim 1 wherein a portion of the steam-cracked gas oilfraction which has been cooled to a temperature in the range of fromabout 350* to about 400*F. is passed through a second heat exchangingzone in order to further generate steam in the range of from about 10 toabout 30 psig.